1. Field of the Invention
The invention relates to a process and device for the wet-chemical treatment of silicon using an etching liquid that contains water, nitric acid and hydrofluoric acid.
2. The Prior Art
One conventional process for the wet-chemical treatment of silicon wafers is etching in an acidic medium. Etching of silicon in an acidic medium takes place in a two-stage reaction:Si+2(O)→SiO2  (1)SiO2+6HF→SiF62−+2H2O+2H+  (2)In the first step, silicon is oxidized by an oxidizing agent to form silicon dioxide, which ultimately in the second step is dissolved by hydrofluoric acid. To achieve this reaction, an etching liquid that contains nitric acid (as oxidizing agent) and hydrofluoric acid is generally used. Additives such as acetic acid (CH3COOH) and/or phosphoric acid (H3PO4) or surfactants are not involved in the reaction but do alter the reaction rate and the roughness of the etched silicon wafer and are therefore often also added.
Due to the water (H2O) formed during the reaction and the consumption of chemicals, the reaction rate (i.e. the etching rate) and the surface roughness of the etched silicon wafer change. By way of example, the presence of water increases the roughness of the etched silicon wafer. Although the reaction rate can be kept constant by stoichiometrically topping up the consumed chemicals (generally by topping up with approximately the same amount as has been consumed by the chemical reactions), the roughness cannot be kept constant. This is because, despite the topping up, the water content of the liquid increases constantly as the amount of dissolved silicon rises.                To counteract this effect, either            1) the etching liquid is discarded and replaced with fresh etching liquid after a certain amount of silicon has been treated, for example after a certain number of silicon wafers have been treated, or    2) more nitric acid and hydrofluoric acid than are stoichiometrically required are topped up, in order to keep the water content at a predetermined level.Variant 2) has the advantage that in this case the roughness of the treated silicon wafers can be kept constant, but at the same time it requires a considerably increased consumption of chemicals. Moreover, in this process too, a new batch (comprising fresh etching liquid) is required from time to time, for example after maintenance work, and consequently fluctuations in the roughness of the treated silicon wafers cannot be avoided altogether.
In the case of variant 1), differences in roughness are within certain limits inevitable, the roughnesses generally being less than the roughnesses produced in variant 2). Advantages are the reduced consumption of chemicals and considerably reduced chemicals disposal costs.
In the case of variant 1), the consumed etching liquid has to be replaced with fresh etching liquid at regular intervals. In the case of etching with fresh etching liquid, the oxidation of the semiconductor material according to reaction equation (1) is effected exclusively by the nitric acid (HNO3) used:2HNO3+Si→SiO2+2HNO2  (3)However, during subsequent etching runs the nitrous acid (HNO2) formed also acts as an oxidizing agent, nitrous acid having a stronger oxidizing action than nitric acid:4HNO2+Si→SiO2+2H2O+4NO  (4)
Therefore, the etching rate and consequently also the geometry of the silicon wafers produced by the etching during the first etching run are different than the subsequent runs. This is a serious drawback of variant 1).
This drawback can be avoided if the etching liquid that contains nitric acid and hydrofluoric acid is activated before first being used by the addition of silicon, for example of silicon waste, in which case nitrogen oxides (NOx) and nitrous acid (HNO2) are formed in accordance with reaction equations (3) and (4). A process of this type is described in U.S. Pat. No. 5,843,322. However, this process has various drawbacks: for example, acid is consumed and water generated as early as during the activation. In addition, particularly if silicon waste with a surface area that is not accurately defined is used (in the case of silicon fragments or in the case of multiple use of silicon wafers), it is difficult to meter the quantity of silicon that reacts and therefore the degree of activation.